Biology, epidemiology

• Conservation, sources d'inoculum
  

Scerotinia sclerotiorum and S. minor have significant saprophytic potential. They can be maintained in the soil for 8 to 10 years thanks to the sclerotia (figure 1) that they produce on the affected organs and / or to the mycelium present in the plant debris abandoned on the plots. In addition, they are polyphagous fungi that can be found on many host plants.

reported S. sclerotiorum has been on more than 400 different plant species, cultivated or weeds. It infects many vegetable crops that rotate with tomato, such as salads, beans, cabbages, peppers, eggplants, many cucurbits, celery, peas, carrots, rutabaga, potatoes. , the sunflower for the industrial tomato A number of weeds harbor it unnoticed.

Less polyphagous, S. minor is still described on more than 90 plant species. In his case, the severity of the attacks is closely correlated with the number of sclerotia present in the soil.

These numerous hosts are capable of multiplying them and of serving as sources of inoculum when they are incorporated, after harvest, into the soil with the sclerotia of these 2 fungi.

Contamination of S. minor occurs mainly through the mycelium from sclerotia found near organs in contact with the ground. These sclerotia must have dried for some time before they can germinate.

Contamination of tomato by plants S. sclerotiorum can occur in the same way. On the other hand, this fungus forms apothecia on its sclerotia. These organs ensure its sexual reproduction and generate numerous asci containing ascospores. Thus, millions of ascospores are released from the apothecia into the air over 2 to 3 weeks; they are the source of airborne contamination, sometimes over several hundred meters. They can only germinate on the leaves in the presence of water from rain, sprinkler irrigation or dew.

• Penetration and invasion

Whatever the nature of the inoculum (mycelium, ascospores), these 2 fungi easily penetrate into living, injured, senescent or dead organs in contact or not with the soil, and invade them quickly. Their mycelia progress into healthy tissue, which they decay using numerous lytic enzymes. For example, Sclerotinia sclerotiorum produces endo- and exopectinases, hemicellulases and proteases. It also synthesizes oxalic acid, which influences both the expression of its pathogenic power and the receptivity of its host.

When ambient humidity permits, these two Sclerotinia spp. produce white mycelium more or less dense and sclerotia on damaged tissue. When crop residues are incorporated into the soil, 70% of the sclerotia are found in the first 8 centimeters of depth.
 

• Sporulation and dissemination

The sclerotia sometimes ensure the transmission of these fungi to other plots, such as they are transported through the soil on plowing tools or plants. As previously reported, unlike S. minor (heterothallic species), S. sclerotiorum (homothallic species) readily produces apothecia (figure 2) , asci and ascospores disseminating , especially when temperatures are low, including between 8 and 16 ° C.

• Conditions favorable to its development

If their thermal optima are slightly below 20 ° C, these two Sclerotinia are able to grow at temperatures between 4 and 30 ° C . They are favored by humid and rainy periods and are particularly fond of tissues that have reached advanced development.

Light soils rich in humus are more conducive to the development of S. sclerotiorum . The latter is sensitive to carbon dioxide, which explains its location in the very first centimeters of the ground. The temperature and humidity conditions of the soil also influence the survival of the sclerotia of these fungi. Apothecia are also formed as a result of rains, thunderstorms, irrigation increasing soil moisture.